Rechargeable (secondary) lithium ion batteries are widely utilized in consumer electronic devices such as cell phones and laptop computers owing, in part, to their high energy density. Rechargeable lithium ion batteries are also useful in power-intensive applications, such as in electric vehicles and power tools. Additional uses for rechargeable lithium ion batteries, such as in energy grid storage, are also considered.
A rechargeable battery stores electrical energy as chemical energy in two electrodes, an anode and a cathode. In a rechargeable lithium ion battery, the anode and the cathode are electrically insulated from one another inside the battery by an electrolyte and typically also by a separator. The separator is permeable to a lithium-ion electrolyte that allows lithium ions (Li+) to pass between the electrodes inside the battery. The electrons (e−) move through an external electronic circuit. The anode and the cathode normally include compounds into which lithium ions and/or lithium atoms may be reversibly inserted. The electrolyte typically contains a lithium salt dissolved in an organic liquid to produce lithium ions. Often the electrolyte contains an organic liquid, such as a carbonate, an ether, a nitrile or a sulfoxide.
When the lithium ion battery is discharged, electrons move from the anode to the cathode passing through an external device, such as a phone, which is powered by the electron flow, i.e. current. The current flowing through the external device can also be of electron vacancies, i.e. holes. Lithium ions move from the anode to the cathode at the same time. When the lithium ion battery is charged, an outside power source, such as a wall socket, supplies the power required for transporting lithium ions through the electrolyte and electrons through the external circuit from the cathode to the anode. Preferably, the lithium formed of the lithium ions and the electrons, combines with, dissolves in, alloys in, or intercalates in a material of the anode. On discharge the flow of ions and electrons is reversed and the lithium combines with, dissolves in, alloys in, or intercalates in a material of the cathode. The same process occurs, but with sodium ions, in a rechargeable sodium ion battery.
Although some uses are not particularly sensitive to the rate at which a battery charges and discharges, many are. For example, a battery that can recharge in an hour is much more practical for an electric vehicle than one that requires several hours. Similarly, a cell phone battery that recharges in 5 minutes is far more convenient that one that requires 30 minutes. Batteries that discharge rapidly provide more power, e.g., acceleration in an electric vehicle, higher torque for a power tool, or transmission power and range to a mobile telephone.
Currently, there are a variety of cathode materials available for lithium ion batteries that can be charged quite quickly. As a result, the charge time for most lithium ion batteries is currently limited by the anode material. Power-related properties are similarly limited. Some anode materials are capable of supporting charge and discharge rates similar to the capabilities of cathode materials, but these anode materials tend to exhibit other problems. For example, Mo3Sb7 and Li4Ti5O12 anodes allow rapid charge and discharge, but at the cost of reducing the voltage of batteries combining these materials with common cathode materials. The power density and the energy density of a discharging battery usually increase linearly with the operating voltage.
Other anode materials exhibit these and other problems. For example, although they provide batteries with high voltages and reasonably quick charge and discharge times, lithium-metal anodes tend to form metal dendrites that cross from the anode to the cathode, resulting in a short circuit within the battery. Yet other anode materials provide for high rates without dendrite formation, but their coulombic capacity, meaning the charge they store per unit volume (volumetric capacity) or per unit mass (gravimetric capacity) is small, making the energy density of the battery low.
Similar issues are encountered with sodium ion batteries, although both cathode materials and anode materials are less developed for such batteries.
New high rate anode materials of high coulombic capacity providing high battery voltages in combination with appropriate cathode materials for lithium ion batteries and sodium ion batteries are needed.